Of fluoroalkanesulfonic acids, for example, trifluoromethanesulfonic acid is a useful material as a raw material for producing lithium trifluoromethanesulfonate, which is used as a lithium cell electrolyte. As the method for producing lithium trifluoromethanesulfonate suitable for cells is shown by the reaction formula (1),2CF3SO3H+Li2CO3→2CF3SO3Li+CO2+H2O  (1)it is obtained by subjecting a high-purity trifluoromethanesulfonic acid to a neutralization reaction by a high-purity lithium carbonate.
In this case, the raw material acid is required to contain impurities, such as sulfuric acid, free fluorine, etc., as little as possible. Thus, it is necessary to reduce these impurities until less than ten ppm. For example, the method of Patent Publication 1 is known as one that is industrially widely conducted as a method for producing fluoroalkanesulfonic acid. As shown in the following scheme, trifluoromethanesulfonyl fluoride is produced by an electrolytic fluorination method, followed by conducting a hydrolysis reaction by heating with a potassium hydroxide aqueous solution to obtain potassium trifluoromethanesulfonate, and then the potassium salt is reacted with excessive 100% sulfuric acid, followed by conducting a distillation to obtain a crude product of trifluoromethanesulfonic acid. Furthermore, the crude product is subjected to a rectification to obtain trifluoromethanesulfonic acid of high purity.
<Patent Publication 1>

There is, however, a problem that the production cost increases since the step for obtaining trifluoromethanesulfonic acid of high purity is long in this method, and since the environmental load is heavy due to the use of organic solvent, etc., too. Furthermore, in the acid decomposition reaction, since the distillation under heating and reduced pressure is conducted after excessively adding concentrated sulfuric acid and fuming sulfuric acid, a pyrolysis reaction of the target product and/or side reactions proceed. This causes a large amount of free fluorine derived from hydrogen fluoride, and its mixing into the fluoroalkanesulfonic acid cannot be prevented. Therefore, it has been impossible to prevent the quality from lowering strikingly and that from having a large amount of free fluorine, etc. Furthermore, there has also been a problem of containing a large amount of sulfuric acid in the fluoroalkanesulfonic acid after the distillation under heating and reduced pressure.
The present applicant has proposed a method with an incorporated recycling step, in which, similar to Patent Publication 1, a mixed aqueous solution of potassium trifluoromethanesulfonate and potassium fluoride is obtained, followed by crystallizing potassium trifluoromethanesulfonate by concentration or adding an alkali, then separating the crystals by filtration, and circulating the filtrate to the gas absorption step, as a method for industrially advantageously producing a fluoroalkanesulfonic acid by simpler apparatus and operation than those in the past (Patent Publication 2).
Furthermore, the present applicant has found a method for producing a fluoroalkanesulfonic acid with a small amount of free fluorine by adding sulfuric acid and silica or a silica compound to a fluorocarbon compound, as a method for obtaining a fluoroalkanesulfonic acid of high purity by removing free fluorine, and has made an application therefor (Patent Publication 3).
Similarly, the present applicant has found a method of adding water during the purification under heating (in the following, it is mentioned as “water addition distillation”, too) in order to solve the mixing of sulfuric acid into the fluoroalkanesulfonic acid after the distillation under heating and reduced pressure and has made an application therefor (Patent Publication 4).